Four objectives are proposed for this research project. The form to be used is the common snapping turtle, Chelydra serpentina. Evidence has accumulated that temperature during incubation is the determinate of sexual differentiation. There appears to be a relatively short critical period during incubation during which this determination occurs. These propositions are to be explored and correlated with stages in differentiation of the gonads and other endocrine organs. The morphology of leucocytes during embryogenesis and in the adult is to be investigated. When variations in control are established, it is proposed to compare controls with blood of experimental animals which are rejecting allografts. After partial extirpation, somites in the turtle behave as a mosaic and the regulation such as described in the chick after complete somite extirpation was not observed. It is proposed to attempt to do complete somite extirpations in turtle embryos to determine whether or not the ability to reconstitute a basic embryonic component is basically the same or different in these two forms. A basic objective is to continue to demonstrate the usefulness of turtle embryos and hatchlings for problems of developmental biology and to facilitate their use by interested colleagues.